Skip to main content

Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

See other formats



reduced to sheet and two test-pieces of 0-5 gram each weighed with the
greatest exactitude", if possible, each piece should consist of a single square
of about 0-5 cm. side. This is placed between the two pieces of silver foil
and the whole wrapped in a piece of white paper or of thin lead foil (allowing
for the weight in calculating the lead to be taken) and placed in the tray
beside the corresponding quantity of lead.

Meanwhile the furnace is started and the cupels, from which the dust
has been blown, introduced. In gold assay there is no danger of sensible
loss owing to volatilisation, so that a rather higher temperature than for
silver may be used. According to T. K. Rose,1 the most suitable mean tem-
perature is about 1070, each 5 above this causing a loss of o-oi on the fine-
ness. When the temperature indicated is reached, each cupel is charged
with the weighed quantity of lead, which rapidly melts, becomes covered
with a layer of oxide and, after a few instants, becomes uncovered, i.e.,
shining. With great care, to avoid spurting, the little parcel of sample
and silver is placed in the cupel. The phenomena of the cupellation are
identical with those observed with silver ; after some time the agitation

FIG. 26

FIG. 27

on the surface is observed, then the iridescence, and finally the bright flashing.
The cupels are then moved towards the door of the muffle, allowed to cool
somewhat and withdrawn, the buttons being detached with a suitable
utensil. With successful cupellation, the buttons should be hemispherical,
shining and white at the upper part and opaque white at the lower. The
button is held in strong pincers, the edges struck with a hammer and the
flat part freed from cupel dust by means of a stiff brush. It is then placed
on a clean anvil and struck alternately on the sides and on the flat part so
as to give it a somewhat elongated form and is next reheated to redness by
leaving it for a short time on a cupel in the front part of the muffle. When
cool, it is rolled to obtain a strip (fillet) about 0-5 mm. thick and of the form
shown in Fig. 26. This strip, bent in two in a smooth curve, is again reheated
to redness for 4-5 minutes on a cupel. When cold, it is twisted into
a spiral round a glass rod so as to obtain almost a tube about 0-5 cm. wide
(cornet), care being taken that the rolls of the spiral do not touch (Fig. 27) ;
this is then subjected to the operation of parting.

Parting. Each of two assay flasks is charged with 25-30 c.c. of nitric
acid (D 1-2) absolutely free from chlorine, nitrous fumes and selenic acid,
this being heated to boiling and the two cornets introduced. The heating
is continued so as to maintain the liquid in gentle ebullition for ten minutes
after evolution of nitrous vapours ceases.2 After a short rest, the liquid

1  Journal of the Chemical Society, 1893, LXIII, p. 707.

2  In assay laboratories, there is usually a special apparatus for the elimination of
the nitrous vapours and acid fumes evolved during the treatment of the cornet with
nitric acid.    It consists of one or two series of 8-10 small bunsen burners, over which
are placed as many flasks with their inclined necks projecting through a rectangular
aperture into a space communicating with an efficient draught chamber.    Betweentc. The